What does a variable mean in an addition expression

In addition or subtraction, a variable represents an unknown number.

Example(s):
1. 20 - x = 17; the variable is 'x'
2. 5 + d = 25; the variable is 'd'

Therefore, you can use any letter to represent a variable. If you don't know what the unknown number is, just pretend that the variable is either a 1 or 0.

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